This application claims the priority of 198 33 147.9-13, filed Jul. 23, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method of adjusting the engine braking performance of a supercharged internal-combustion engine provided with a variably adjustable turbine geometry for changing the effective turbine cross-section.
DE 196 37 999 A1 describes an internal-combustion engine whose exhaust gas turbocharger can also be used for an engine braking. The turbine of the exhaust gas turbocharger has a variably adjustable turbine geometry, which in the engine braking operation is changed into a ram position reducing the nozzle cross-section of the turbine. An increased exhaust back pressure is thereby achieved. The exhaust gas flows at a high flow rate through ducts between guide blades of the turbine geometry and acts upon the turbine wheel which drives the compressor, whereupon an excess pressure builds up in the intake system.
As a result of the foregoing, the cylinder is acted upon by an increased supercharging pressure on the input side. On the output side, an excess pressure exists between the cylinder outlet and the exhaust gas turbocharger. This excess pressure counteracts a blowing-off of the air compressed in the cylinder by way of decompression valves into the exhaust gas system. In the engine braking operation, during the compression and push-out stroke, the piston must carry out decompression work against the high excess pressure in the exhaust gas system, whereby a strong braking effect is achieved.
Furthermore, DE 196 37 999 A1 teaches subjecting the engine braking performance to an automatic control in order to either be able to maintain a constant desired speed or brake the vehicle to zero within the shortest time possible. For example, in a danger situation in the latter situation, a maximal braking performance is required.
In principle, the directional control of the vehicle must also be taken into account during all braking maneuvers. This applies particularly to fully loaded utility vehicles and during critical road conditions, particularly in snow and ice.